1. Field of the Invention
This invention relates to a control apparatus for an electrically driven power steering system mounted on a fork lift truck.
2. Related Art Statement
An illustrative electrically driven power steering system mounted on a fork lift truck is disclosed for example in a Japanese Laid-open Utility Model Publication No. 107682/1986 corresponding to U.S. Pat. No. 4,676,331.
In the above steering control apparatus, when the steering wheel of the fork lift truck is driven into partial rotation, the torque produced by the manipulation is sensed by a torque sensor and steering torque signals are generated to the controller from a potentiometer mounted to the torque sensor. The controller is designed and constructed for generating a drive power to a steering assist servo motor in accordance with prescribed steering assist characteristics determined on the basis of the torque signals generated from the potentiometer, and the steering assist force being generated from the servo motor to the steering mechanism. In the conventional controller shown in FIG. 1, the signal corresponding to the steering force TH as shown in FIG. 1 is generated from a potentiometer 101 and amplified in an amplifier 102. The amplified signal is modulated and controlled in, for example, a pulse width modulation control circuit (PWM) 102, which the generates a drive output to the servo motor 104 on the basis of the pulse width modulation control and the feedback control. The servo motor 104 then generates a steering assist force to a steering mechanism 105 on the basis of the linear steering assist characteristics as indicated in FIG. 2.
The above described electrically driven power steering contrtol device generates a steering assist force TM with linear steering assist characteristics, as indicated in FIG. 1. It is noted that, in this connection, the larger load WL placed on the fork lift truck 51, the more the rear wheels 52 tend to be urged upward as shown in FIG. 10, whereby the load WR thereof becomes smaller, and a driving torque for a driving thereof becomes smaller. Thus the conventional electrically driven power steering control device has a drawback that, since the steering assist characteristics remain constant despite changes in the load weight, the operator will feel the handling too light in steering for a larger load WL or for a higher traveling speed of the fork lift truck, so that the operator will have only an unstable feeling in operating the truck.